1. Field of the Invention
The present invention relates to industrial furnaces, and more particularly to a method of uniformly distributing heat within an industrial furnace and of decreasing the quantity of NOx in the combustion products.
2. Description of the Related Art
Today industrial furnaces are widely used for melting and for heat treating metals, for example. Many of those furnaces use one or several burners of the conventional type, which are fed by a fuel such as propane, oil, natural gas, or the like, and fed also with an oxidant. In many cases, air is used as the oxidant.
Such furnaces can be of various sizes. It is often desirable to maintain an even temperature distribution within the furnace, which can be difficult to achieve in the case of large furnaces, since the convective heat transfer within the furnace becomes less effective as the volume of the furnace increases. That problem can be solved, by way of example, by rotating the furnace body during operation.
In certain cases, an extra supply of oxidant can be arranged in a furnace, in addition to the oxidant supplied through the burner itself. For example, an extra oxidant supply can be achieved by the use of a supply pipe, arranged at a distance from the burner, through which oxidant can flow into the furnace. Such a supply pipe is referred to herein as a lance, and the procedure for bringing oxidant to flow into a furnace body through such a lance is referred to herein as lancing.
If the thus-lanced oxidant is an oxidant having at least 85 weight % oxygen gas that is lanced in that manner, for example, and if it is directed into the furnace body at an angle that makes the stream of oxidant intersect a point at or near the flame of the burner, various phenomena arise. For instance, it is possible to control the temperature distribution of the flame in a way so that it becomes rotationally asymmetric. Such phenomena can be utilized in order to control the temperature profile of the flame, for example, so that the flame becomes warmer near a blank to be heated, and less warm near a furnace wall.
In order to achieve stoichiometric balance, the amount of oxidant lanced in that way and the amount of oxidant fed from the burner must be balanced against the amount of supplied fuel. That balance is achieved by decreasing the amount of oxidant fed by the burner, whereby a stoichiometric deficit is introduced in the flame immediately outside the nozzle of the burner. However, that stoichiometric deficit is balanced against the lanced oxidant at the place where the lanced stream of oxidant intersects the flame.
A problem regarding industrial furnaces with burners using air as the oxidant is that their efficiency is relatively low, and their heat output is relatively high. That means that the heat output is relatively high in terms of fuel amounts, as compared to the useful yield that in fact is usable for heating the furnace volume. One reason for that is that thermal energy is used for heating the nitrogen content of the oxidant air.
Another problem is that the flame of a burner using air as the oxidant produces relatively large amounts of combustion products in the form of NOx compounds, as a result of the elevated flame temperatures. That is not desirable, since NOx compounds negatively affect the environment, and since there often are limitations present for the volumes of NOx compounds that can be produced in various industrial processes.
As a solution to those problems, so-called oxyfuel burners have been suggested for use in industrial furnaces. In oxyfuel burners, oxygen gas is used as the oxidant. Such burners offer a higher efficiency at a lower heat output, whereby less fuel is needed to achieve the same yield in terms of heating within the furnace. Also, the amount of NOx compounds produced is decreased.
However, there is a problem in that there is a substantial cost involved to exchange an air-fed burner with an oxyfuel burner in an industrial furnace. Furthermore, it is difficult to maintain high temperature uniformity in the entire furnace volume when such a burner is used in large industrial furnaces, because convection decreases as a consequence of the lesser amounts of flue gases that are produced as a result of the lower operation effects for oxyfuel burners. The result is uneven furnace temperature distribution, with the associated uneven quality of production.
Another suggested solution, on the other hand, is to increase the share of oxygen gas in the supplied air. However, that has proven to result in other problems.
The present invention solves the above problems.